A tremendous variety of devices used today rely on actuators of one sort or another to convert electrical energy to mechanical energy. Conversely, many power generation applications operate by converting mechanical action into electrical energy. Employed to harvest mechanical energy in this fashion, the same type of actuator may be referred to as a generator. Likewise, when the structure is employed to convert physical stimulus such as vibration or pressure into an electrical signal for measurement purposes, it may be referred to as a sensor. Yet, the term “transducer” may be used to generically refer to any of the devices.
A number of design considerations favor the selection and use of advanced dielectric elastomer materials, also referred to as “electroactive polymers” (EAPs), for the fabrication of transducers. These considerations include potential force, power density, power conversion/consumption, size, weight, cost, response time, duty cycle, service requirements, environmental impact, etc. As such, in many applications, EAP technology offers an ideal replacement for piezoelectric, shape-memory alloy (SMA) and electromagnetic devices such as motors and solenoids.
An EAP transducer comprises two thin film electrodes having elastic characteristics and separated by a thin elastomeric dielectric material. When a voltage difference is applied to the electrodes, the oppositely-charged electrodes attract each other thereby compressing the polymer dielectric layer therebetween. As the electrodes are pulled closer together, the dielectric polymer film becomes thinner (the z-axis component contracts) as it expands in the planar directions (the x- and y-axes components expand).
Examples of EAP devices and their applications are described in U.S. Pat. Nos. 7,394,282; 7,378,783; 7,368,862; 7,362,032; 7,320,457; 7,259,503; 7,233,097; 7,224,106; 7,211,937; 7,199,501; 7,166,953; 7,064,472; 7,062,055; 7,052,594; 7,049,732; 7,034,432; 6,940,221; 6,911,764; 6,891,317; 6,882,086; 6,876,135; 6,812,624; 6,809,462; 6,806,621; 6,781,284; 6,768,246; 6,707,236; 6,664,718; 6,628,040; 6,586,859; 6,583,533; 6,545,384; 6,543,110; 6,376,971 and 6,343,129; and in U.S. Patent Application Publication Nos. 2008/0157631; 2008/0116764; 2008/0022517; 2007/0230222; 2007/0200468; 2007/0200467; 2007/0200466; 2007/0200457; 2007/0200454; 2007/0200453; 2007/0170822; 2006/0238079; 2006/0208610; 2006/0208609; and 2005/0157893, the entireties of which are incorporated herein by reference.
Many EAP transducer operate at high voltages, e.g., in the range from about 0.5 kV to about 50 kV. Like any high voltage device, EAP transducers are susceptible to partial discharge. Energy moves from a region of high electrical potential to a region of lower electrical potential, e.g., from the high voltage electrode to the ground electrode. A partial discharge occurs when a small quantity of charge (i.e., picoCoulombs) does not bridge the entire space between the electrodes. Areas of steep gradients in electrical potential favor partial discharges. These include electrode edges, projections extending from an electrode, cracks internal to the electrode, and gas filled microvoids within the dielectric material. Generally, the smaller the radius of curvature of the electrode geometry, the lower the voltage necessary to initiate and maintain partial discharge. Put another way, the smoother the electrode surfaces, the less likely partial discharge will occur.
Partial discharges through air are particularly damaging to dielectric elastomer transducers. The discharge may be from the electrode into the air, which serves as a virtual ground—a phenomenon commonly called “corona discharge.” Alternately, the charge may pass though the air as it jumps from the electrode to an adjacent region of the dielectric surface. In either case, movement of the charge through air is energetic, producing fluorescence, ionized gas, and temperatures within the arc on the order of thousands of degrees Celsius. The ionized gas reacts to produce corrosive materials like ozone and nitrogen oxides that yield nitric acid under conditions of high humidity. These reactive species, in combination with the high temperatures present within the electrical arc, erode the electrode and dielectric materials and can shorten the life span of a transducer.
The inventors of the subject invention are not aware of any prior art dielectric elastomer/electroactive polymer transducers that are designed to inhibit or suppress partial discharge and corona. Thus, it would be highly advantageous to fabricate and provide EAP transducers having such a feature.